Determination of quality of bonding between a conductive ball and a conductive pad within an IC package

ABSTRACT

The quality of bonding between a conductive ball and a conductive pad of a lead frame strip of an IC package are determined by etching the conductive ball from the conductive pad, and analyzing the bottom view of the conductive ball. The conductive ball is comprised of a first conductive material and a conductive pad is comprised of a second conductive material. The conductive ball is bonded to the conductive pad by formation of an intermediary material formed from the first conductive material of the conductive ball and the second conductive material of the conductive pad. The lead frame strip is immersed within an etching solution such that the intermediary material is etched between the conductive ball and the conductive pad until the conductive ball may be decoupled from the conductive pad. The conductive ball has a first color in areas where the intermediary material was not abutting the conductive ball and has a second color in areas where the intermediary material was abutting the conductive ball when the conductive ball was bonded to the conductive pad. The intermediary material was abutting an intermediary material area of the conductive ball when the conductive ball was bonded to the conductive pad. A magnified image of the intermediary material area on the conductive ball is captured by a microscope with the intermediary material area being substantially centered within the magnified image while the conductive ball is decoupled from the conductive pad. The magnified image of the intermediary material area on the conductive ball is analyzed by a data processor to determine quality of bonding between the conductive ball and the conductive pad.

This is a divisional of an earlier filed copending patent applicationwith Ser. No. 09/602,733 filed on Jun. 26, 2000 now U.S. Pat. No.6,193,134, for which priority is claimed. This earlier filed copendingpatent application with Ser. No. 09/602,733 is in its entiretyincorporated herewith by reference.

TECHNICAL FIELD

The present invention relates generally to IC (integrated circuit)packages, and more particularly, to a method and system for determiningthe quality of bonding between a conductive ball and a conductive pad ofan IC (integrated circuit) die within an IC package by etching theconductive ball from the conductive pad and analyzing a magnified imageof the bottom of the conductive ball.

BACKGROUND OF THE INVENTION

An IC (integrated circuit) die typically is housed within an IC(integrated circuit) package having leads that are coupled to conductivepads on the IC die for providing connection to nodes of the integratedcircuit. Referring to FIG. 1, a cross sectional view of a lead framestrip 100 of an IC package includes an IC die 102 mounted on a die framedap 104. The die frame dap 104 is coupled to the lead frame strip 100via tie bars 108. The structures of the lead frame strip 100 are part ofan IC package and are known to one of ordinary skill in the art of ICpackage manufacture.

Further referring to FIG. 1, a lead interconnect 112 of the lead framestrip 100 is coupled to a conductive pad on the IC die 102 for providingconnection to a node of the integrated circuit of the IC die 102. Aconductive ball 114 is bonded to a conductive pad 116 on the IC die 102.The conductive ball 114 is coupled to the lead interconnect 112 via awire 118. FIG. 1 is a cross sectional view along a line A—A of a topview of the lead frame strip 100 of FIG. 2.

The performance of the integrated circuit of the IC die 102 isdetermined by the quality of bonding between the conductive ball 114 andthe conductive pad 116. Referring to FIG. 3A, the conductive ball 114 isbonded to the conductive pad 116 using an ultrasonic and heating processto form an intermediary material 120 that bonds the conductive ball 114to the conductive pad 116, as known to one of ordinary skill in the artof IC package manufacture. In the ultrasonic and heating process, theintermediary material 120 is formed from a first conductive material ofthe conductive ball 114 and a second conductive material of theconductive pad 116. For example, if the conductive ball 114 is comprisedof gold and the conductive pad 116 is comprised of aluminum, then theintermediary material 120 is comprised of an intermetallic alloy(Au_(x)Al_(y)) formed from the gold of the gold ball 114 and thealuminum of the aluminum pad 116.

FIG. 3A illustrates an example of poor bonding between the conductiveball 114 and the conductive pad 116. In FIG. 3A, a relatively low amountof material of the conductive ball 114 and the conductive pad 116 havebeen used to form a low amount of the intermediary material 120. Withsuch low amount of intermediary material 120 bonding the conductive ball114 to the conductive pad 116, the conductive ball 114 may have highresistance poor contact with the conductive pad 116. Such highresistance degrades the speed performance of the integrated circuitwithin the IC die 102.

FIG. 3B illustrates an example of good bonding between the conductiveball 114 and the conductive pad 116. In FIG. 3B, a relatively highamount of material of the conductive ball 114 and the conductive pad 116have been used to form a high amount of the intermediary material 120.With such high amount of intermediary material 120 bonding theconductive ball 114 to the conductive pad 116, the conductive ball 114may have low resistance good contact with the conductive pad 116. Suchlow resistance enhances the speed performance of the integrated circuitwithin the IC die 102.

Because the performance of the integrated circuit within the IC die 102depends on the quality of bonding between the conductive ball 114 withthe conductive pad 116, the quality of bonding is monitored duringmanufacture of IC packages. Referring to FIG. 4A, in the prior art, across section along line B—B is made to result in the cross sectionalview of FIG. 4B of the intermediary material 120 between the conductiveball 114 and the conductive pad 116. Such a cross sectional view isanalyzed to determine the quality of bonding between the conductive ball114 and the conductive pad 116. For example, a resin material is formedaround the conductive ball 114 and the conductive pad 116, and theconductive ball 114 and the conductive pad 116 are polished from theside inward to the cross section B—B to result in the cross sectionalview of FIG. 4B.

For consistent analysis, the cross section B—B should consistently bethrough the center of the conductive ball 114 as multiple IC packagesare examined. However, polishing down to the exact center of theconductive ball 114 in the prior art is difficult to control. Referringto FIG. 5A, if the cross section B—B is not at the center of theconductive ball 114, then the cross sectional view of FIG. 5B resultswith an inaccurate representation of the amount of intermediary material120 between the conductive ball 114 and the conductive pad 116. Thus,the quality of the bonding between the conductive ball 114 and theconductive pad 116 cannot be consistently determined with the polishingmethod of the prior art.

In addition, a cross section of the intermediary material 120 may not bean accurate representation of the quality of bonding between theconductive ball 114 and the conductive pad 116. For example, if a voidor a contaminant is present within the intermediary material 120, and ifthe cross section is not through such a void or contaminant, then thecross section alone may not represent the poor contact between thecontact ball 114 and the contact pad 116 from such a void orcontaminant.

Nevertheless, because bonding between the conductive ball 114 and theconductive pad 116 in an IC package determines the performance of theintegrated circuit, accurate and consistent determination of the qualityof bonding is desired.

SUMMARY OF THE INVENTION

Accordingly, in a general aspect of the present invention, the qualityof bonding between a conductive ball and a conductive pad of a leadframe strip of an IC package is determined by etching the conductiveball from the conductive pad and analyzing the bottom of the conductiveball.

The conductive ball is comprised of a first conductive material and aconductive pad is comprised of a second conductive material. Theconductive ball is bonded to the conductive pad by formation of anintermediary material formed from the first conductive material of theconductive ball and the second conductive material of the conductivepad.

In one embodiment of the present invention, the lead frame strip isimmersed within an etching solution such that the intermediary materialis etched between the conductive ball and the conductive pad until theconductive ball may be decoupled from the conductive pad. The conductiveball has a first color in areas where the intermediary material was notabutting the conductive ball and has a second color in areas where theintermediary material was abutting the conductive ball when theconductive ball was bonded to the conductive pad. The intermediarymaterial was abutting an intermediary material area of the conductiveball when the conductive ball was bonded to the conductive pad. Amagnified image of the intermediary material area on the conductive ballis captured by a microscope with the intermediary material area beingsubstantially centered within the magnified image when the conductiveball is decoupled from the conductive pad.

The intermediary material area appears as the second color within themagnified image, and an area of the conductive ball wherein theintermediary material was not abutting the conductive ball appears asthe first color within the magnified image. The magnified image of theintermediary material area on the conductive ball is analyzed by a dataprocessor to determine quality of bonding between the conductive balland the conductive pad.

The present invention may be used to particular advantage when the dataprocessor calculates a percentage of the intermediary material area to atotal area of the conductive ball within the magnified image todetermine the quantity of bonding between the conductive ball and theconductive pad.

In another embodiment of the present invention, the etching solution iscomprised of about 30% (weight/volume) sodium hydroxide (NaOH) at atemperature of about 80° Celsius, and the lead frame strip is immersedin the etching solution for a time period of about 1 minute foroptimized etching of the intermediary material between the conductiveball and the conductive pad.

In this manner, the quality of bonding between the conductive ball andthe conductive pad may be consistently and accurately determined for aplurality of IC packages during manufacture of a high quantity of ICpackages.

These and other features and advantages of the present invention will bebetter understood by considering the following detailed description ofthe invention which is presented with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross sectional view of a lead frame strip of an IC(integrated circuit) package having a conductive ball bonded to aconductive pad of an IC (integrated circuit) die;

FIG. 2 shows a top view of the lead frame strip of FIG. 1;

FIG. 3A shows a cross sectional view of the conductive ball bonded tothe conductive pad with poor bonding;

FIG. 3B shows a cross sectional view of the conductive ball bonded tothe conductive pad with good bonding;

FIG. 4A shows a top view of the conductive ball and the conductive padwith a cross section line being across the center of the conductiveball;

FIG. 4B shows the cross sectional view of the conductive ball bonded tothe conductive pad along the cross section line of FIG. 4A, for analysisof the quality of the bonding between the conductive ball and theconductive pad in the prior art;

FIG. 5A shows a top view of the conductive ball and the conductive padwith a cross section line being far from the center of the conductiveball;

FIG. 5B shows the cross sectional view of the conductive ball bonded tothe conductive pad along the cross section line of FIG. 5A, for analysisof the quality of the bonding between the conductive ball and theconductive pad in the prior art;

FIG. 6 illustrates a bath of etching solution for etching the conductiveball from the conductive pad of the lead frame strip, according to anembodiment of the present invention;

FIG. 7 illustrates the cross section of the conductive ball and theconductive pad after etching of the intermediary material between theconductive ball and the conductive pad such that the conductive ball maybe decoupled from the conductive pad, according to an embodiment of thepresent invention;

FIG. 8 illustrates the cross sectional view of the lead frame strip ofFIG. 1 with decoupling of the conductive ball from the conductive pad,according to an embodiment of the present invention;

FIG. 9 illustrates cutting away of the die frame dap holding the IC diethereon from the lead frame strip of the IC package such that an imageof the bottom of the conductive ball may be captured, according to anembodiment of the present invention; and

FIG. 10 illustrates processing of the image of the bottom of theconductive ball as captured in FIG. 9 for determining the quality ofbonding between the conductive ball and the conductive pad, according toan embodiment of the present invention.

The figures referred to herein are drawn for clarity of illustration andare not necessarily drawn to scale. Elements having the same referencenumber in FIGS. 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 refer to elementshaving similar structure and function.

DETAILED DESCRIPTION

Referring to FIG. 6, for determining the quality of bonding between theconductive ball 114 and the conductive pad 116, the lead frame strip 100is immersed within a bath 200 of an etching solution. Referring to FIGS.6 and 7, the intermediary material 120 is etched between the conductiveball 114 and the conductive pad 116 by the etching solution of the bath200. For example, when the conductive ball 114 is comprised of gold andthe conductive pad 116 is comprised of aluminum, the intermediarymaterial 120 is comprised of an intermetallic alloy (Au_(x)Al_(y))formed from the gold of the gold ball 114 and the aluminum of thealuminum pad 116, as known to one of ordinary skill in the art of ICpackage manufacture.

For such materials, in a preferred embodiment of the present invention,the quality of bonding between the conductive ball 114 and theconductive pad 116 may be consistently and accurately determined whenthe etching solution of the bath 200 is comprised of about 30%(weight/volume) sodium hydroxide (NaOH) and when the lead frame strip100 is immersed within the bath 200 at a temperature of about 80°Celsius for a time period of about 1 minute. The bath 200 is heated tothe predetermined temperature of 80° Celsius under control of atemperature controller 202.

Referring to FIGS. 7 and 8, after the intermediary material between theconductive ball 114 and the conductive pad 116 is etched, the conductiveball 114 is decoupled away from the conductive pad 116. For the leadframe strip 100 having a plurality of conductive balls on a plurality ofconductive pads, each of the plurality of conductive balls is decoupledfrom the plurality of conductive pads. However, just an exampleconductive ball 114 and an example conductive pad 116 are illustrated inFIG. 8 for clarity of illustration.

Referring to FIGS. 8 and 9, after the conductive balls are decoupledfrom the conductive pads of the IC die 102, the die frame dap 104 is cutaway from the lead frame strip 100 at the tie bar 108. In this manner,the bottom of the conductive ball 114 is floating with the conductiveball 114 also being coupled to the lead frame strip 100 via the wire 118such that an image of the bottom of the conductive ball 114 may easilybe captured. Referring to FIGS. 9 and 10, a microscope 206 is used tocapture a magnified image 208 of the bottom of the conductive ball 114.Microscopes that capture magnified images are known to one of ordinaryskill in the art of IC package manufacture.

Referring to FIG. 10, the magnified image 208 includes an area 210 ofthe conductive ball 114 wherein the intermediary material 120 was notabutting the conductive ball 114 when the conductive ball was bonded tothe conductive pad. The magnified image 208 also includes anintermediary material area 212 wherein the intermediary material 120 wasabutting the conductive ball 114 when the conductive ball was bonded tothe conductive pad. The microscope 206 is controlled such that themagnified image 208 has the intermediary material area 212 substantiallycentered within the magnified image 208.

The intermediary material area 212 appears as a first color in themagnified image 208. The area 210 of the conductive ball 114, outside ofthe intermediary material area 212 wherein the intermediary material 120was not abutting the conductive ball 114, appears as a second color inthe magnified image 208. For example, when the conductive ball 114 iscomprised of gold and the conductive pad 116 is comprised of aluminumand the intermediary material is comprised of a gold and aluminum alloy(Au_(x)Al_(y)), the first color of the intermediary area 212 appearsgray in the magnified image 208. The second color of the area 210 of theconductive ball 114 outside of the intermediary material area 212appears gold in the magnified image 208.

The intermediary area 212 and the area 210 of the conductive ball 114outside of the intermediary material area 212 within the magnified image208 are then analyzed to determine the quality of bonding between theconductive ball 114 and the conductive pad 116. For example, in oneembodiment of the present invention, the magnified image 208 may beprocessed by a data processor 220 which may be a computer for example asknown to one of ordinary skill in the art of IC package manufacture. Thedata processor 220 may include an image processing application fordetermining the percentage of the intermediary material area 212 to thetotal area of the conductive ball 114 within the magnified image 208.One example of such an image processing application is Image-Prosoftware available from Media Cybernetics in Silver Spring, Md., andthis Image-Pro software is commercially available to one of ordinaryskill in the art of IC package manufacture.

In this manner, the percentage of the intermediary material area 212 tothe total area of the conductive ball 114 within the magnified image 208is a quantifiable measure of bonding between the conductive ball 114 andthe conductive pad 116. A higher percentage indicates a higher quantityof bonding between the conductive ball 114 and the conductive pad 116.In addition, by viewing the whole area of the intermediary material 120,voids or contaminants present within the intermediary material 120 maybe detected. The quality of bonding between the conductive ball 114 andthe conductive pad 116 may be monitored in this manner duringmanufacture of IC packages such that corrective action may improve thequality of poor bonding.

In addition, applicants have by experimentation optimized the processparameters for etching the intermediary material 120 in the bath 200 ofFIG. 6 to include an etching solution comprised of about 30%(weight/volume) sodium hydroxide (NaOH) at a temperature of about 80°Celsius and an etching time period of about 1 minute. Such etchingconditions optimize etching a proper amount of intermediary material 120between the conductive ball 114 and the conductive pad 116 such that thepercentage of the intermediary material area 212 to the total area ofthe conductive ball 114 within the magnified image 208 may beconsistently and accurately determined for a plurality of IC packages.

If the amount of intermediary material 120 between the conductive ball114 and the conductive pad 116 is under-etched, then the conductive ball114 may not be decoupled from the conductive pad 116. On the other hand,if the amount of intermediary material 120 between the conductive ball114 and the conductive pad 116 is over-etched, then the intermediaryarea 212 may be a smaller size resulting in determination of aninaccurate percentage of the intermediary material area 212 to the totalarea of the conductive ball 114 within the magnified image 208.

The foregoing is by way of example only and is not intended to belimiting. For example, the present invention is illustrated for anexample lead frame strip of an IC package with an example conductiveball on a conductive pad. The present invention may be used with anyother types of lead frame strips of IC packages, as would be apparent toone of ordinary skill in the art of integrated circuit manufacture fromthe description herein. In addition, the material of any structurespecified herein is by way of example only.

Furthermore, as will be understood by those skilled in the art, thestructures described herein may be made or used in the same wayregardless of their position and orientation. Accordingly, it is to beunderstood that terms and phrases such as “top,” “side,” and “bottom” asused herein refer to relative location and orientation of variousportions of the structures with respect to one another, and are notintended to suggest that any particular absolute orientation withrespect to external objects is necessary or required.

The present invention is limited only as defined in the following claimsand equivalents thereof.

We claim:
 1. A system for determining quality of bonding between aconductive ball comprised of a first conductive material and aconductive pad comprised of a second conductive material within an ICpackage, the system comprising: an intermediary material formed fromsaid first conductive material of said conductive ball and said secondconductive material of said conductive pad for bonding said conductiveball to said conductive pad when said conductive pad is bonded to saidconductive ball; a bath of etching solution for etching saidintermediary material between said conductive ball and said conductivepad such that said conductive ball is decoupled from said conductivepad, said conductive ball having a first color in areas where saidintermediary material was not abutting said conductive ball and having asecond color in areas where said intermediary material was abutting saidconductive ball when said conductive ball was bonded to said conductivepad; and wherein said intermediary material was abutting said conductiveball on an intermediary material area of said conductive ball when saidconductive ball was bonded to said conductive pad; a lead frame strip ofsaid IC package with said conductive ball being coupled via a wire tosaid lead frame strip of said IC package, and wherein said pad isdisposed on an IC (integrated circuit) die supported by a die frame dapof said IC package, and wherein said die frame dap with said IC diethereon is cut away from said lead frame strip such that said conductiveball is floating via said wire that is coupled to said lead frame strip;a microscope for capturing a magnified image of said intermediarymaterial area on said conductive ball with said intermediary materialarea being substantially centered within said magnified image while saidconductive ball is decoupled from said conductive pad; and wherein saidintermediary material area appears as said second color within saidmagnified image and wherein an area of said conductive ball wherein saidintermediary material was abutting said conductive ball appears as saidfirst color within said magnified image; and a data processor includinga computer-readable medium having computer-executable instructions foranalyzing said magnified image of said intermediary material area onsaid conductive ball to determine quality of bonding between saidconductive ball and said conductive pad.
 2. The system of claim 1,wherein said etching solution is comprised of about 30% (weight/volume)sodium hydroxide (NaOH), and wherein said system further comprises: atemperature controller for maintaining a temperature of said etchingsolution at a temperature of about 80° Celsius.
 3. The system of claim1, wherein said data processor includes a computer-readable mediumhaving computer-executable instructions for calculating a percentage ofsaid intermediary material area to a total area of said conductive ballwithin said magnified image to determine quantity of bonding betweensaid conductive ball and said conductive pad.
 4. The system of claim 1,wherein said conductive ball is comprised of gold (Au).
 5. The system ofclaim 4, wherein said conductive pad is comprised of aluminum (Al), andwherein said intermediary material is comprised of a gold and aluminumalloy (Au_(x)Al_(y)).